Sodar systems as described in the patent applications listed above use pulse compression of acoustic signals (preferably acoustic chirps) that have a relatively wide bandwidth (when compared to conventional short pulse SODAR systems) to obtain vertical wind speed and direction as well as large and small scale turbulence.
Use of relatively wideband acoustic chirps in Sodar systems (to realize full advantages of pulse compression) provides many potential advantages over conventional short pulse single frequency systems including increased gain and better resolution but also introduces several problems. The problems arise directly from the use of wide bandwidth acoustic chirps and the resulting Doppler shift of return signals.
Known problems resulting from the use of wideband acoustic chirps include:    i) increasing range error as wind speed increases due to Doppler shift; and    ii) loss of resolution as wind speed increases due to differential Doppler shift.    iii) differential attenuation of the atmosphere at different frequencies such that a single wideband chirp would incur greater loss at higher frequencies and the advantage of using a wide band chirp would be lost.    iv) Decorrelation of a return signal when reflection of the return signal is from different patches of turbulence.
As may be seen from the above, it is the very attribute of a pulse compression signal that provides increased system gain and good resolution at very low wind speeds that also reduces performance of the Sodar system in higher wind speed. Use of a wideband chirp may also be of limited value because of a loss of processing gain due to greater attenuation of the chirp at higher frequencies especially at lower humidity.
The parameters of wide bandwidth pulse compression chirp Sodar typically include a transmitted audio frequency chirp that increases linearly from about 1 kHz to 3 kHz over a period of several seconds. Chirp transmit signals that linearly decrease from about 3 kHz to 1 kHz (reverse chirp) may also be used with only a change in the signs of the equations being necessary to describe this operation. For the purpose of describing operation of the system, only linearly increasing (or decreasing) chirps will be described herein. In order to obtain a useable range for the Sodar, a receive period may be several seconds longer than the period of the transmitted signal, so that complete chirps may be received from a distance which is determined by the receive period.
For instance if a chirp is transmitted for 5 seconds and a range of 1000 m is required then the receive time must allow for complete chirp signals to travel up and back, a distance of 2000 m. This implies that total receive time for such a system should be about 6 seconds (assuming a speed of sound of 340 m/s) plus transmit time giving a total receive time of 11 seconds. The range R is obtained from time using the equationR=ctR/2  (1)wherein c is the speed of sound in air and tR is the range time being the time taken for a transmitted signal to travel up and back, after the end of a transmit pulse.
The transmitted signals are reflected back from atmospheric discontinuities such as turbulence during daytime or by molecular scattering at night. The returned signals thus include a continuous set of the transmitted chirp signals reflected from the atmosphere which is a continuous target.
The method of the present invention may include transmitting into the atmosphere acoustic chirps such as 200 Hz each, having a relatively wide bandwidth in total (eg. 2 kHz), for a period of say up to but not limited to 10 seconds.
The chirp signals returned from the atmosphere are received at some later time and may be passed through a matched filter as shown in FIG. 1. Estimates of the received signal phase and amplitude may be obtained from real and imaginary parts associated with the received signal.
Doppler phase information is indicative of radial wind velocity and may allow signal amplitude and phase outputs to be shifted to their correct location to compensate for range errors.
Resolution errors may be corrected by taking the first received Doppler phase signal at each of a number of range segments. The first received Doppler phase may be used to estimate the amount of frequency shift in the received signal at any given range so as to construct a new chirp that is representative of the actual Doppler shift. The new chirp may then be used to reprocess the received signals at any given range to improve the resolution of the system.
Chirps having an increasing or decreasing frequency may be used without any loss of generality in the above description. Indeed, an increasing frequency chirp may be transmitted on one beam with a decreasing frequency chirp transmitted on another beam simultaneously. The arrangement may be further exploited by using chirps of different frequencies. Both forward and reverse chirps may be transmitted simultaneously on several beams to greatly speed up acquisition of wind data. For instance, 4 chirps may be transmitted simultaneously in the following manner on 4 different beams, East 1 kHz to 2.5 kHz, West 2.5 kHz to 1 kHz, South 3 kHz to 4.5 kHz, North 4.5 kHz to 3 kHz. In this way a complete set of wind vectors can be obtained in one transmit-receive cycle instead of having 4 transmit-receive cycles if just one increasing frequency chirp is used.
The processing of the simultaneous multiple chirps may include passing each forward and reverse frequency segment through a respective matched filter and correcting range error for each output from each respective matched filter. The chirps transmitted on substantially opposite beams may then be subtracted after correcting the range error.
This arrangement is particularly advantageous as simultaneous measurement of all wind vectors minimizes error due to measuring at different times wherein the wind may have changed between measurements.
By subtracting opposite vectors (North-South and East-West) after error correction, horizontal wind components (North South component and East West component) may be obtained. Actual wind speed and direction may then be easily obtained.
Furthermore, if opposite beams are added a vertical wind speed is obtained for each of the North South and East West beams. As two sets of vertical wind speed are obtained it may be a simple matter to compare each of the North South and East West vertical wind speeds to provide a quality control process, if the North South and East West beams are working correctly the vertical wind speed from each of the North South and East West additions should be the similar.
A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.
Where the terms “comprise”, “comprises”, “comprised” or “comprising” are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.